The present invention relates generally to ion implantation systems, and more particularly to methods and apparatus for improved ion acceleration in an ion implantation system.
In the manufacture of semiconductor devices, ion implantation is used to dope semiconductors with impurities. A high energy (HE) ion implanter is described in U.S. Pat. No. 4,667,111, assigned to the assignee of the present invention, which is hereby incorporated by reference as if fully set forth herein. HE ion implanters are used for deep implants into a substrate in creating, for example, retrograde wells. Such implanters typically perform implants at energies between at least 300 keV and 700 keV. Some HE ion implanters are capable of providing ion beams at energy levels up to 5 MeV.
Referring to FIG. 1, one implementation of high energy ion implanter 10 is illustrated, having a terminal 12, a beamline assembly 14, and an end station 16. The terminal 12 includes an ion source 20 powered by a high voltage power supply 22. The ion source 20 produces an ion beam 24, which is provided to the beamline assembly 14. The ion beam 24 is then directed toward a target wafer 30 in the end station 16. The ion beam 24 is conditioned by the beamline assembly 14, which comprises a mass analysis magnet 26 and a radio frequency (RF) linear accelerator (linac) 28. The mass analysis magnet 26 passes only ions of an appropriate charge to-mass ratio to the linac 28.
The linac 28 includes a series of accelerating stages or modules 28a-28n, each of which further accelerates ions beyond the energies they achieve from prior modules. The accelerator modules 28a-28n in the implementation of FIG. 1 are individually energized by dedicated, fixed-frequency RF amplifiers and resonator circuits (not shown). The linear accelerator modules 28a-28n in the high energy ion implanter 10 individually include an RF amplifier, a resonator, and an energizable electrode, wherein the resonators operate at a fixed frequency in order to accelerate ions of the beam 24 to energies over one million electron volts per charge state.
The accelerator 28 of FIG. 1 may be adapted to efficiently accelerate various ion species through adjustment of the relative phase between adjacent accelerator modules 28a-28n. However, the adjustments in the individual accelerator modules 28a-28n must be made carefully in order to provide for proper acceleration of ions through the entire accelerator 28. Thus, sophisticated controls and/or trial and error methodologies are commonly employed in order to tune such multi-variable accelerator systems 28 for specific acceleration energies, and for specific ion species. In addition, the provision of multiple fixed-frequency amplifiers associated with individual accelerating stages 28a-28n is costly and such dedicated amplifiers and associated resonator circuits occupy a significant amount of space in conventional ion implantation systems. Thus, there remains a need for improved ion acceleration apparatus and methodologies to facilitate low cost, simplified ion implantation systems.
The present invention is directed to an ion accelerator for use in an ion implantation system, as well as methodologies for accelerating ions in such a system, which reduce or overcome the problems and shortcomings found in conventional accelerators. In particular, an ion accelerator is provided, comprising a plurality of energizable electrodes energized by a variable frequency power source or amplifier, in order to accelerate ions from an ion source. The employment of a variable frequency power source allows the ion accelerator to be adapted to accelerate a wide range of ion species to desired energy levels for implantation onto a workpiece. The single power source reduces the cost and complexity of the ion accelerator and associated controls compared with conventional accelerators, and additionally reduces the size thereof. The invention further includes methodologies for accelerating ions in an ion implantation system, which may be employed to achieve performance and cost advantages over conventional methodologies.
One aspect of the invention provides an ion accelerator for accelerating ions traveling along a path in an ion implantation system. The accelerator includes one or more accelerating stages, each stage having one or more energizable electrodes and a variable frequency RF system, such as a variable frequency power source and an associated variable frequency resonator. The accelerator stage or stages may comprise constant potential (e.g., grounded) electrodes interleaved between the energizable electrodes, where the RF system energizes all the energizable electrodes in phase with one another. Alternatively, alternating energizable electrodes can be connected to a first RF system terminal, with the remaining electrodes connected to a second terminal, for instance, such that adjacent energizable electrodes are energized 180 degrees out of phase.
The accelerator may also comprise a variable frequency buncher stage located upstream of the initial accelerating stage to provide bunched ions thereto. Reliability in such an implementation may be improved in accordance with the present invention, since only two RF systems are required (e.g., such as a high power RF system for the accelerating stage and a lower power RF system for the buncher stage). Moreover, the reduced number of independent RF systems (e.g. power sources and resonators) simplifies associated control systems and may reduce the time and effort required to tune ion implantation systems. Where multiple accelerating stages are used, or where a buncher stage is provided, the stages are operable at the same frequency or one stage may be operated at a harmonic of the frequency of another stage. In addition, the relative phasing between multiple stages, and/or between accelerating stages and a buncher stage may be controlled at a fixed relationship, or may be adjustable.
Because a single variable frequency power source is used to energize a series of energizable electrodes, the system cost and size are significantly reduced compared with conventional ion accelerators having an RF system for each energizable electrode. In addition, the invention provides an accelerator which is much easier to tune and control, particularly where an ion implantation system is used to implant different ion species at different energy levels. Thus, the system complexity is reduced along with the complexity of associated controls, whereby reduced setup and/or tuning time is achieved. In addition, where previous systems may have been limited in their ability to support a wide range of ion species and energy levels (e.g., due to the complexity involved in tuning the individual resonators and fixed frequency amplifiers), the present invention provides an accelerator with fewer system variables, which is adaptable to support a wide range of ion species and energy levels.
The variable frequency power source, moreover, may be adjustable to provide RF energy to the energizable electrodes in a frequency range appropriate to support commonly used ion species and acceleration energy levels. For instance, the power source may be adjustable in a range of from about 1 to 10 times a given frequency, such as from about 4 MHz to about 40 MHz. The invention comprises any number of such energizable electrodes in a given accelerating stage. The invention may thus provide significant cost and space savings over existing high energy ion implantation systems and linear accelerators.
Another aspect of the invention provides an ion implantation system comprising an ion accelerator as described above having one or more energizable electrodes energized with a variable frequency power source, as well as an ion source providing an ion beam to the accelerator, an end station adapted to position a workpiece so that accelerated ions impact the workpiece, and a controller operative to control the accelerator and/or other system components. The implantation system may further comprise a dedicated ion buncher located upstream of the initial accelerating stage. Yet another aspect of the invention involves a method of accelerating ions in an ion implantation system. The method comprises providing a plurality of energizable electrodes spaced from one another in series along a path, and applying an alternating potential of a controlled frequency and amplitude to the plurality of energizable electrodes using a variable frequency RF power source in order to create alternating electric fields along the path, whereby ions are accelerated along the path.
To the accomplishment of the foregoing and related ends, the invention comprises the features hereinafter fully described and particularly pointed out in the claims. The following description and the annexed drawings set forth in detail certain illustrative aspects and implementations of the invention. These are indicative, however, of but a few of the various ways in which the principles of the invention may be employed. Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings.